Acoustic microscope

ABSTRACT

An acoustic microscope is disclosed having an acoustic lens formed of fused metal and glass. A high speed analogue to digital converter and a pulse generator/receiver are also disclosed. Automated V(z) curve measurement produces quantitative elastic properties of the specimen. (C) and (B) scans are sliced and a simulated three-dimensional image is formed therefrom.

This invention relates to the fields of ultrasonic testing, imaging,analysis and processing, and more particularly relates to an acousticmicroscope. Such microscopes are widely used in nondestructive testing,observation and analysis in the material, electronics, mechanics andmedical fields.

GB 2311858 relates to an ultrasonic imaging system. Ultrasonic wavesare, generated by laser beams. Reflected waves are detected andconverted into digital signals. The digital signals are processed usinga synthetic aperture focussing technique in order to obtain focussedimages.

Ultrasonic testing has been known for several decades. The testingdevices most widely used are industrial A scan flaw detector, medical Bscan ultrasonic scanner, C scan ultrasonic imagery machine and otheradvanced products, such as acoustic microscopes.

A, B and C scan modes are the three basic modes of ultrasonic imagingclassified under the pulse echo techniques. When an ultrasonic wave inpulse form impinges on a discontinuity it will be reflected and thereflected pulse can be recorded. This reflected pulse carriesinformation on the discontinuity. This is quite similar to the formationof echo and hence the name pulse echo.

The A scan is the simplest form of ultrasonic imaging. It just recordsthe reflected pulse or signal and is one dimensional and no images areformed.

The B scan or brightness mode scanning, provides a two dimensional,cross sectional reflection image of the object that is scanned. It givesthe side view of the object. A B scan image is formed by sweeping anarrow acoustic beam through a plane and positioning the received echoeson a display such that there is a correspondence between the displayscan line and the direction of acoustic propagation in the material.Generally the same transducer is used to both send and receive theacoustic signals. A fundamental feature of a B scan image is that one ofthe dimensions is inferred from arrival time of each of a short acousticpulse as they reflect from structures along a (presumed) straight-linepath. Signals received from structures close to the transducer arriveearlier than signals received from structures far from the transducer.The other (transverse) dimension is obtained by moving the transducer(either physically by mechanical means or apparently by electronicmeans) so that a different straight line path through the object isinterrogated by another short acoustic pulse. This process is continueduntil the entire object region of interest is scanned. Some means oftracking the propagation path through the object is required in order tounambiguously define the image.

The C scan provides a two-dimensional orthographic image of an object.It gives the top view of the object. Unlike, the B scan, where onedimension of the image is inferred from the arrival time of an acousticpulse, time plays no primary role in either of the two image dimensionsof a C-scan. In a reflection C-scan, the time of arrival plays asecondary role in that it determines the distance of the image planefrom the transducer; in a transmission C-scan, time plays no rolewhatsoever. A C-scan image resembles images obtained with x-rayfluoroscopy; hence, the images tend to look more familiar than acorresponding B-scan and are often more readily interpretable.

Because acoustic microscopes usually employ high-frequency acousticwaves, accurately-focusing lenses and high-precision mechanical scanningsystems, the images taken have high accuracy. Using a computer, digitalinformation obtained from tests using an acoustic microscope aredigitized to save and display. In this manner, it is easy to observe theresult. Also, an acoustic microscope can perform different functions,such as testing, observation and analysis using image processing andother signal processing techniques.

The conventional acoustic microscope contains four principal parts:pulse generation and signal detecting circuit 1, lens 3, mechanicalscanning system 5, and also control and display 7 (FIG. 1). The pulsegeneration and signal detecting circuit 1 includes main pulse generator9, pulse divider 11, high-frequency oscillator 13, amplifier 15, PINmodulator 17, circulator 19, matching network 21, amplifier and signaldetector 23, pulse amplifier 25, sample and hold circuit 27, sample andhold pulse source 29, video frequency amplifier 31, filter 33 and pulseoscilloscope 35. The mechanical scanning system 5 comprisesthree-dimensionally adjustable workstation 37, XYZ axes moving andscanning. workstation 39, step-motor and position sensors 41. Thecontrol and display 7 comprises computer 43, keyboard 45, mouse (notshown), A/D interface board 47, parallel interface board 49 and monitor51. The sample 53 is mounted on the three-dimensionally adjustableworkstation 37 and is coupled to the lens 3 by a suitable couplingliquid 55, such as water.

Continuous microwave signals are generated by a microwave source(components 9 to 15) and pass through modulator where they are convertedto modulated pulse signals. These signals go through circulator 19,matching network 21 and are converted to modulated acoustic pulses whenthey pass to the lens 3, and are focused by the lens 3 into the sample53 and reflected from the inhomogeneous parts of the sample. Acousticsignals are reflected back to lens 3, through a transducer and convertedto modulated electrical pulses. These pulses go through circulator 19,high frequency amplifier, then through detecting, amplifying, samplingand holding (components 23 to 33) to A/D converter 47, and are saved inthe memory of computer 39 and displayed by the monitor 51.

Conventional acoustic microscopes have some important deficiencies:

-   1. The pulse time for gathering data is fixed, so only one section    image is obtained for each scan (i.e. the plane perpendicular to the    direction of travel of the acoustic waves). Each time one C scan    image is obtained only. It is not possible to obtain C scan image of    several cross-sections simultaneously.-   2. It is not possible to obtain the structure of the plane    containing the acoustic wave, i.e. the B scan and of course not    possible to have A scan information. Hence it is very difficult to    obtain a three-dimensional image.

In 1994, the applicant designed a multi-purpose imaging system that canobtain A, B and C scan images at the same time (FIG. 2). The inspectionprinciple is similar to that described in relation to FIG. 1, and likeelements are designated with the same reference numerals. The onlysignificant difference is that, when above the specimen (X1, Y1)position, the lens sends a series of acoustic pulses to the specimen,and at the same time sends a series of data gathering pulses, the delaytimes of each of data gathering pulses relative to main pulses varyingfrom small to large. Provided that each delay time between each datagathering pulse and main pulse is small enough, the series of datagathering pulses can be detected from the amplitude of reflected signalsfor a given point at different depths. By collecting all the reflectedacoustic signals, an A scan image for this point (X1, Y1) is obtained.If the collected signals are used to modulate the brilliance of variouspoints on this line, a B scan image is obtained. After the scan of XYsurface of the sample is completed, all necessary information isobtained for internal three-dimensional structure, and hence a completethree-dimensional image is formed. This test system is an obviousimprovement compared with the system shown in FIG. 1, because it canobtain A, B and C scan images, and internal three-dimensional structurefor the specimen. The main disadvantage is that the speed is too low. Ittakes more than one hour to get a series of three-dimensional data.

In order to improve the scanning accuracy of the scanning acousticmicroscope (SAM), Professor Kino of Stanford University, devised thesystem described in U.S. Pat. No. 4,503,708 (Reflection AcousticMicroscope for Precision Differential Phase Image)—see FIG. 3. Theworking process of the system of the U.S. patent is similar to theapplicant's proposal described above. The difference is that, in theU.S. patent, not the amplitude but the phase of the reflected acousticsignals is measured. Here special hardware (lock-in amplifier and phasedetector) are used to test the phase of reflected acoustic signals. Theadvantage of this system is high accuracy. It can examine the changingof depth for the membrane. The accuracy can reach about 10 angstrom. Butthis system is. much more complex than the system in FIG. 1. It isexpensive and difficult to operate.

Another prior system is disclosed in U.S. Pat. No. 4,577,504 (Hitachi).

An object of the present invention is to mitigate the deficiencies ofthe four prior systems mentioned above. The system of the presentinvention may quickly obtain A, B, C scan images and display athree-dimensional image. The system may perform phase testing andimaging. Also the noise can be reduced. The system may obtain theinternal character of the sample and differentiate with the model. Thehardware may be very simple, only including the basic elements of theprevious SAM for the collection of the original data and processing withsoftware.

The present invention is defined in claims 1, 11, 12, 13 and 15.

For a better understanding of the present invention, an embodiment willnow be described, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of the main structure of a conventional SAM;

FIG. 2 is a block diagram of a prior SAM of the present applicant;

FIG. 3 is a block diagram of the prior SAM of U.S. Pat. No. 4,503,708;

FIG. 4 is a block diagram of the present invention;

FIG. 5 is a circuit diagram of the pulse generator/receiver;

FIG. 6 is a block diagram of the A/D converter;

FIG. 7 is a cross-sectional view through an acoustic lens;

FIG. 8 shows the processing of the reflected waveform;

FIG. 9 is a schematic showing the theory of curve fitting;

FIG. 10 is a schematic showing the theory of phase testing;

FIG. 11 shows an A scan waveform;

FIG. 12 shows A, B, C scan images;

FIGS. 13, 14, 15 and 16 show example C scan images;

FIG. 17 shows the elimination of a fixed pulse from an A scan waveform;

FIG. 18 is a flow chart showing the removal of useless fixed pulses;

FIG. 19 shows the reduction in pulse width of an A scan waveform;

FIG. 20 shows the elimination of the multireflections from A and C scanwaveforms;

FIG. 21 illustrates the cubic voxel in the Marching Cube Algorithm;

FIG. 22 shows an image of the internal structure of a flipchip;

FIG. 23 shows an image of the bottom surface of a ceramic plate;

FIG. 24 shows an image of the interior delamination of the structureshown in FIG. 12(c);

FIG. 25 shows images of the interior structure of an aluminum sample;and

FIG. 26 shows the software frame of the invention.

The principal components of the acoustic microscope are shownschematically in FIG. 4. A pulse generator 100 produces high speed andhigh voltage pulses. The output of the pulse generator 100, which willbe a series of 400 V, 10 ns duration pulses, is applied to T connector102. Electrical pulses are sent from the T connector 102 to an acousticlens 104, under which a sample 106 is located. A transducer 105 ismounted on the lens 104. For frequencies higher than 10 Mz, thetransducer is ground from 36° Y cut LiNbO₃ single crystal. Forfrequencies lower than 10 MHz, the transducer is made from PZTpiezoelectric ceramics. A liquid (not shown) couples the lens 104 to thesample 106. The coupling liquid is typically distilled water. The sample106 is mounted on a scanner 108, which is moved by stepmotor driver 110.The sample can be moved along three perpendicular (XYZ) axes.

The SAM uses a multidimension adjustment structure. It has two sets ofangle adjusters, separately mounted on the transducer support, and thesample stage. At the transducer support, there is a focus adjuster. Thehorizontal scanning of sample by the ultrasonic wave bundle is achievedby a mechanical method. The grating style scanning of the sample isachieved by using a PC to control the step motor to drive the highprecision moving stage. Due to the broad frequency range of the systemthere is a big difference in the sensitivity requirement. Ad lowfrequencies, the step motor 110 moves in steps of three beats. But athigh frequencies, the step motor 110 moves in more minute intermediatesteps.

The pulse excites the lens 104 and generates an acoustic wave with afrequency of 25-200 MHz that is incident on the sample 106. Due toinhomogenities in the sample 106, acoustic waves are reflected back tothe lens 106. The reflected waves are converted into electrical signalsand pass from the T connector 102 to pulse limiter 112, thence tohigh-frequency, high-gain amplifier 114. The electrical signalsgenerated by the lens will comprise a series of high-frequency modulatedpulses.

Components 100, 102, 112, 114, 118 and 120 comprise a pulsegenerator/receiver. The pulse generator/receiver may be in the form of aPC card. The pulse generator/receiver has a simple structure with adirect current high voltage electrical source and a high quality Marxtype avalanche electronic circuit.

The output of the amplifier 114 of the pulse generator/receiver passesto A/D converter 116 via peak detector 118 and pulse amplifier 120. TheA/D converter is in the form of a PC card, and is a high speed devicewith a sampling rate of over 1 GSPS and a differentiation rate of 8bits. It can digitize un-modulated signals with frequencies above 100MHz. The digitized output passes to computer 122, such as a PC with PII300 MHz processor.

The ultrahigh speed A/D digitises the reflected wave signal and allowsthe software to carry out signal processing and display functions. Itsatisfies the requirement of realtime. acquisition. The principles ofthe ultrahigh speed A/D card are given in block diagram as follows inFIG. 5. The integrated circuit chosen for the A/D card is the AD9058,which has a speed of 50 MSPS. The AD9058 has parallel processingcharacteristics, and the opposite phase function is employed to increasespeed to 100 MSPS. An equal time multiple phase acquisition technique isused. Software controls an AD9501 integrated circuit to produce a seriesof delayed pulses. Multiple data acquisitions replace a single dataacquisition. Therefore, the actual data acquisition speed is the singledata acquisition speed multiplied by the number of times data isacquired.

The computer 122 runs software for interpreting the signals reflectedfrom the sample 106. The results are displayed on monitor 124. A clocksignal generator 126 synchronises the computer 122, the pulse generator100 and the A/D converter 116. The computer 122 controls the stepmotordriver 110 and scanner 108 using its parallel output part.

The computer 122 has control of the overall system shown in FIG. 4. Thesoftware run by the computer replaces many of the hardware components ofthe prior art described above. This reduces the cost and size of thesystem, whilst allowing additional functions to be provided. The onlyfunctions performed by hardware are essentially signal generating,amplifying and digitizing.

A circuit diagram of the pulse generator/receiver is shown in FIG. 5.The pulse generator/receiver comprises a Marx type avalanche circuit 300including four 2N5551 transistors. This avalanche circuit iscommercially available. Control circuits 302 which control the initialfunctions and increase the performance of the pulse generator/receiverare also provided. Receiver and amplifier stage 304 receives andamplifies the sound wave signal resulting from reflection of sound wavesfrom the acoustic lens 104. The output of the stage 304 is prepared fortransmission to the A/D converter 116. Software running of the computer122 supplements the functions performed by the circuitry shown in FIG.5, and this allows the circuitry to be simpler than that which isconventionally employed.

The acoustic lens 103 is a glass and metallic ball surface transducertype acoustic lens. The structure of the lens is shown in FIG. 7, whichis a cross-sectional view. The lens comprises a molybdenum rod 200surrounded by glass 202. The glass and metal may be intermelted andintersealed. The part of the lens 104 which is nearest the specimen 106,in use, has an indentation 204 corresponding in shape to part of asphere. On the lower surface of the lens 104, including the indentation204, a piezoelectric layer 206 is formed. The layer 206 may be of zinc,oxide, lithium niobate or silica. A metallic, for example gold, film 208is formed over the piezoelectric layer 206. The layers 206 and 208comprise an electrode. outer protective layer 210 is then formed overfilm 208. The protective layer 210 is a high-hardness, grindingresistant film, such as silicon nitrate or granite formed by a MPCVDtechnique.

The assembly described in the preceding paragraph is located in ahousing 212 formed of, for example, copper. The housing may includedownwardly-extending claws 214 which extend beyond the lower surface ofthe assembly to protect the assembly in case of contact with thespecimen 106 or any other object. The copper of the housing 212 may bejoined with the gold of the film 208 to form the electrode.

A coaxial connecting socket protects the upper surface of the lens.

The diameter of the rod 200 is less than the diameter of the indentation204. The sound wave produced by the lens 104 is a spherical wave andwill be convergent and will not produce interference waves. Comparedwith a sapphire flat surface transducer type acoustic lens, there is oneless reflection surface so losses are reduced. Also, because thematerials used are much cheaper, the cost of the lens is significantlyreduced. The glass/metal structure is relatively easily damaged, butthis disadvantage is mitigated by the provision of the protective layer210 and the claws 214.

The functions performed by the acoustic microscope can be summarised asfollows:

Start ⇓ Pulse Generator Produces High Frequency Pulses to Excite theTransducer to Produce the Ultrasonic Waves ⇓ Ultrasonic Waves areFocussed by the Acoustic Lens ⇓ Focussed Sound Beam Impinges onto theSample and is Reflected ⇓ Reflected Sound Wave Received by the AcousticLens ⇓ Representative Electrical Signal Passed to the High FrequencyAmplifier, Peak Detector and Pulse Amplifier then to the A/D ConverterCard which will Digitise the Signal ⇓ Processing of the Digitised Signalby Software to Remove Noise and Unwanted Pulses ⇓ Lens is Scanned overthe Whole Surface of the Sample Horizontally in Two Dimensions Using theStep Motor and Mechanical Scanning System and the above Steps areRepeated ⇓ Display the Data Collected as Two Dimensional Image on theMonitor ⇓ Process the Digitised Data Using 3D Algorithm and VolumeRendering Software to Produce a 3D Image ⇓ End

To analyse a sample, the sample 106 is moved so that the lens 104 iscentred over a particular point (coordinate value X, Y). The PC controlsthe A/D card to send out an excitation signal. It excites the electroniccircuit to produce 300 V high speed pulse. This excites the acousticlens to produce a high frequency ultrasonic beam. It will be reflectedat the sample's interface or at the uneven part within the sample. Thereflected wave will be received by the acoustic lens, and transformedinto electrical signal (as shown in FIG. 8). After amplification, thereflected signal will be transmitted to the ultrahigh speed A/D card tobe converted into a digital signal. Then the software will be used forprocessing to reduce noise and to carry out wave inspection and wavefiltering.

The A/D card transmits a signal, which is reflected and has a waveformas shown in FIG. 8(a). Then the more effective wavelet transform methodwill be selected for noise reduction. With Mallat high speedcomputation, the result after noise reduction is shown in FIG. 8(b).Then the Akima method is used to carry out wave selection and wavefiltering. The results are shown in FIG. 8(c). Using software for noisereduction, wave selection and wave filtering, can produce a bettereffect, reduce hardware and easily achieve ultrawide band wavefiltering.

The signals after analysis are stored in the computer. It displays atthe same time the A scan waveshape of the sample along its depthdirection, and the B scan and C scan after processing. The software willexcite the scanning mechanical system to move to the next point on thesample and repeat the above procedure.

Scanning will occur in the X direction. The lens is moved to the nextposition of the specimen with coordinate value X2Y1, and the analysingand storing steps are repeated. The sample then moves to X3Y1 . . .XnY1. After finishing scan of each line, the sample moves from Y1 to Y2,and then completes the Xn . . . X2 X1 scan. At each point the sametesting, processing, saving and displaying steps occur. The process isrepeated with scanning Y2 to Y3, and to Y4, Y5 . . . Ym. At each valueof Y, X will scan for a line.

The phenonmenon of Rayleigh wave travel through a medium using a wavegenerated by an acoustic microscope is described in the report“Microwaves, Acoustics and Scanning Microscopy” by C. F. Quate,appearing in the Proceedings of the Rank Prize International Symposiumon Scanned Image Microscopy.

The V(z) curve is applied, in the present embodiment to evaluatequantitative elastic properties of the specimen 106.

The computer 122 is programed to enable the automated measurement of theV(z) curve. Here the v refers to the video or envelope detected signalthat is used to modulate the brightness of the image, and z refers tothe amount by which the specimen surface is displaced from the focalplane of the lens. By convention the focal position is designated z=0,and displacement of the speciment away from the lens is taken aspositive. The process of the decreasing the separation of the lens andspeciment relative to the focal distance is often referred to asdefocusing. The most interesting phenonmena occur at negative defocusand are closely related to the elastic properties of the specimen. Theelastic property of the specimen can be determined by measuring the V(z)curve and using the relation${V_{R}(z)} = {V_{0}\left\{ {1 - \left\lbrack {1 - \frac{V_{0}}{2f\quad{\Delta z}}} \right\rbrack^{2}} \right\}^{\frac{1}{2}}}$

Where V_(R) is the Rayleigh wave speed, V₀ is the speed of the couplingfluid, f is the frequency and Δz is the period of oscillations in thecurve. The elastic properties of an isotropic specimen are completelycharacterized by two independent engineering elastic constants, namely Eand υ, the Young's Modulus-and Poisson's ratio respectively. These twoconstants are related to the Rayleigh wave speed by the followingequation.$E = {2\left( {1 + \upsilon} \right){\rho_{R}^{2}\left\lbrack \frac{1 + \upsilon}{0.87 + {1.12\quad\upsilon}} \right\rbrack}^{2}}$

Where ρ is the density of the specimen. Thus, the Young's Modulus of thespecimen can be determined by measuring V_(R) if the density and thePoisson's ratio are Known.

A Fast Fourier Transform is performed on the measured V(z) data toobtain data z. The program is written in Matlab. From a discreteamplitude plot, data z can be obtained.

To perform V(z) curve testing, the lens 104 is located above certainposition XY of the specimen 106. The Z value, the distance between lensand specimen, is adjusted to an expected value, electrical pulses areinitiated, reflected acoustic signals are tested and saved into memory.By multiplying the phase time differences between the specimen surfaceand reflected signals of lens surface, both of which have the same phasewith sound velocity in water, then Z, the distance between the surfaceand lens, is obtained. The maximum value of V1 is obtained from testingthe reflected acoustic signals. Using Z as horizontal coordinate and Vas vertical coordinate, point V1Z1 is plotted the lens scanned in Zdirection. The distance between the lens and specimen is changed to Z2.The same method as before is used to test reflected voltage value V2 atpoint Z2. Point Z2V2 is then plotted. The drive lens is then againscanned in Z direction to Z3. V3 is obtained. This procedure is repeateduntil the lens moves to Zn, and voltage value Vn is obtained. Then onthe monitor of computer a V(z) curve is plotted. In the computer memory,pairs of V(z) data are saved. Then V(z) analysis software is initiated.

To obtain depth information for each point on the is sample the maximumvalue of the signal and the corresponding time coordinate at which itoccurs are determined. The software applied curve fitting to obtain asmooth curve using the maximum point curve fitted, and displays thecurve on the monitor (FIG. 9). This is the detecting wave of reflectedsignals, also called normal A scan image. With this data B and C scanimages are formed.

When phase information of the sample is needed, reflected acoustic wavescarrying signals gathered above are used, taking certain periods ofcertain layer generating acoustic waves as reference, to calculate thechase difference from the same period another layer generating reflectedacoustic waves, and then obtain the phase image for this layer (FIG.10).

When model identification or low noise is needed, a method, such aswavelet transform or a neural network, is used to reduce the noise oridentification model for the reflected waves carrying signals gatheredabove

When this has been completed the A, B and C scans will be displayed, theC scan image showing depth of the specimen. Original acoustic waves,processed pulse width and phase information of all points of internalthree-dimensional structure are saved in the memory of the computer.This information mast be used for further processing, completing modelidentification and three-dimensional imaging. The results will displayon the monitor with manuscript, data or images, or save in disk, CD orcan be printed out.

Similar to any scanning image formation system, the image formationprocedure is based on the convolution of the objective function and thepoint spread function of the image formation system. Due to the factthat we are aiming at 3D image formation, the SAM's image formationequation is as follows:${U_{d}\left( {\frac{x_{d}}{M},\frac{y_{d}}{M},\frac{z_{d}}{M}} \right)} = {{\underset{v}{\int{\int\int}}}^{h}\left( {x_{0},y_{0},z_{0}} \right){R\left( {{x_{3} - x_{0}},{y_{3} - y_{0}},{z_{3}\quad - z_{0}}} \right)}{\mathbb{d}x_{0}}{\mathbb{d}y_{0}}{\mathbb{d}z_{0}}}$Where

-   -   h=point spread function of image formation system,    -   R=sample's reflection coefficient.

The above equation is different from ordinary image formation equation.The usual equation only considers the 2D scan of the probe along thesample's surface and treats image formation process as the 2Dconvolution of the objective function and the image system's pointspread function. In reality the acoustic lens not only has 2D aberrationin the horizontal direction, in the depth direction also exists theeffect of pulse spread due to reaction from the electrical system pulse.This is also a type of aberration. That is, in reality, the systempossesses a 3D aberration. Hence the system point spread function is a3D function. The image formation equation is the 3D convolution of the3D point spread function and the objective function. During A scanning,the independent variables X₀, Y₀ are automatically cancelled, thevoltage sent from the transducer will be the one D convolution of thesample's reflected function along the depth and the pulse reaction ofthe inspection electrical circuit. During B scanning, one of theindependent variables of X₀, Y₀ (e.g. X₀) will be automaticallycancelled. The output of the transducer output along time (or along z)and y will be the 2D convolution of the pulse reaction of the inspectionelectronic circuit and the 2D point spread function h(y₀, z₀) formed bythe aberration of the acoustic lens along the y direction. During Cscanning, the independent variable z₀ is automatically cancelled. Thedistribution source of the transducer output along X₀, Y₀ is theconvolution of the point spread function h(X₀, Y₀) formed by theaberration of the acoustic lens along the (X₀ Y₀) direction and theobjective reflection function. During 3D image formation, theindependent variables X₀ Y₀ Z₀ all exist. The spatial distribution ofthe transducer output is the resulting 3D convolution of the 3D pointspread function in the broader sense h(X₀,Y₀,Z₀) and the objectivefunction R(X_(s),Y_(s),Z_(s)). Beside this, the multiple reflection ofthe sound wave during measurement and the regular reflections caused bythe existence of the non working model of the lenses also have importantinfluences on the image formation.

The above analysis is the theoretical basis of the research in 3D imageformation. It is pointed out here that in general ultrasonic inspectionapplies to the interior of the samples. The point spread function notonly depends on the frequency, but is also related to the transducerstructure, type of coupling liquid, sample material, shape of the sampleand its depth.

In the present system, the scanning of the ultrasound beam along the x,y direction is accomplished by mechanical scanning. The z axis scanningis accomplished using scanning from the accumulation of time. In orderto obtain sufficient accurate data acquisition, one needs to satisfysample acquisition theorem. To enable the results of data acquisition tobe close to reality, one needs as many acquisition points as possible.That is, the data sampling rate of the A/D card must be as high aspossible. This will enable the waveform recovered after digitisation tobe as close to the waveform of the original signal. According to thesampling theorem, acquisition time interval Δ t±≦½ fn, fh is the portionof the highest frequency among the acquired signals or A/D cardacquisition sampling rate fs =1/Δt≧2 fn. In the applicant's experience,in order to obtain an accurate waveform one needs fs≧10fh. The A/D cardacquisition sampling rate is fs≧1GSPS. Hence it is able to performaccurate acquisition of ultrasound signals above 100 MHz.

During even higher acquisition rate to acquire data, the data acquiredwill occupy greater storage space, and reeds longer processing time.Fortunately, the computation speed, storage and software level has beencontinuously increasing

By way or example, sample inspections will now be described.

During the sample inspection, one places the sample onto the samplestage and adds the couplant. Then the PC and software are initialised.The inspection frequency, scanning limit, speed and acquisition depthare selected. When the inspection has started, the computer display willshow the A scan waveform. Also one can enlarge any required section ofthe waveform to see the details. The A scan waveform shown on the PCdisplay is given in FIG. 11.

During the inspection, the A scan waveform and typical data will beshown at the lower part of the PC monitor display (FIG. 12). The upperright portion of the PC display will show the B scan image and the upperleft portion will display the C scan image. The typical results areshown below.

FIG. 13 shows the C scan image of the internal structure of the threepower triode tubes.

FIG. 14 shows C scan images of Flip Chip's multi-layers at differentdepths.

FIG. 15 shows C scan images of the bottom layer structure of themetallic ceramic base plate.

FIG. 16 shows C scan images of the ceramic metallic welding structuresof the three different samples. The sample after dissection confirmsthat the above images are correct and of high precision. The images showthat the system can also inspect nonflat surface samples.

The software for SAM's 3D data visualization can be divided into threemodules: preprocessing of data, projection, and drawing and display.

1. Preprocessing Data

After standardising the data, one follows a fixed format to store them,and performs preprocessing on these data. According to the imageformation of SAM, one needs to perform the following preprocessing stepsbefore the projection transformation.

a. Elimination of Fixed Pulse

The acoustic lens after high speed pulse excitation will produce aseries of useless fixed pulses mixed within the reflected wave signal.This fixed pulse has to be eliminated during data preprocessing. FIG.17(a) shows the useless fixed pulse of the lens and FIG. 17(b) is the Ascan waveform that includes the useless pulse. The A scan waveform afterthe elimination of the useless pulse is shown in FIG. 17(c).

The technique of wavelet transform is used to eliminate the uselessfixed pulses. As the specimen is raster scanned, all the readings can bearranged into a straight line and a one D wavelet transform performed onthem. To do this the readings acquired from the SAN have to be stored inthe computer's memory first. After which wavelet transform and denoisingwill then be performed on them. The program performs one D wavelettransform, amplitude thresholding and reconstruction is written suing aC++ compiler. In this program, the Haar scaling function and wavelet areused. Their scaling coefficients formed the respective filter banks. Thesignal samples are used as the first set of scaling functioncoefficients. For scales high enough the samples serve as very goodapproximation to the first set of scaling function coefficients. This isapparent to the Haar scaling function as it is a step function. But itapplies to other families of scaling function as well. There are fourfunctions in this program, two of which to do the wavelet transform ofthe signal. One does the amplitude thresholding. The last reconstructsthe signal using the set of lower scale scaling function coefficientsand all the other sets of wavelet coefficients. The memory allocation ofthe arrays and display of results are done in the main program.

The procedure for removing the useless fixed pulse is shown in the flowdiagram of FIG. 18.

b. Reduction of Pulse Width

The sample's surface and the interior delamination's surface are verythin boundaries. The ideal form of reflected sound signal from thatsurface is the f pulse. But due to the limitations of the electroniccircuit bandwidth, the reflected pulse has a fixed width as shown inFIG. 19(a). To overcome this, one can use signal Processing to compressthe signal's pulse width. The result after compression is shown in FIG.19(b).

c. Elimination of Multi Reflections

The ultrasonic wave can produce multi reflections between the tworeflecting surfaces which have high reflection coefficients. The addingof these multi reflection pulses to the reflected pulse will cause ghostimages, as shown in FIG. 20(a). The first reflection in the figure isthe reflection from the sample's surface. The second reflection is fromthe inhomogeneities within the sample. The third reflection is thesecond reflection from the sample's surface. This reflection overlapswith the reflection from the sample's bottom, forming ghost images. FIG.20(c) shows the C scan image which has ghost images. One can follow theknowledge of acoustics, to identify the position of the reflected peakand then investigate in the reverse direction, to eliminate thosereflected peak which have grater influence, and thus eliminate the multireflections. FIG. 20(b) and (d) show the A scan and C scan images afterthe multi reflection elimination.

Besides the above processing method, the traditional methods of spatialfiltering (such as average value filtering and mid value filtering) alsoare effective.

2. Projection

Projection is the nucleus of visualization technology. The visualizationof the SAM data belongs to the field of 3D scalar field technology.Scalar field visualization can be divided into the categories of sectionsurface reconstruction, equal value surface visualization and bodydrawing technology, after considering image formation result andefficiency and the selection of equal value surface visualizationtechnology.

a. Equal Value Surface Visualization

An equal value surface is a series of spatial curved surfaces. Thefunction F(x,y,z) on the curved surface has the given fixed value.Within the spatial graph, each point preserves the sampling valueF(x_(I), y_(I), z_(k)) at the graphic unit (x_(I), y_(I), z_(k)). Foreach given fixed value Ft the equal value surface is the series ofcurved surfaces formed by all the points sFt={(x,y,z):F(x,y,z)=Ft}.

The surface reconstruction method has been used as a way to extractequal value surfaces; It uses the boundary lines of the sectional imageas the basis. It is a type of sectional surface reconstruction method.The precision of the triangular plate used in this method is not sogood. It is unsuitable for use in the highly fluctuating scalar field.With the increase in the capability of the PC, we have selected the moreeffective Marching Cube algorithm to extract equal value surface.

b. Marching Cube Algorithm

The slicing of single scan images into layers of images of the variouslayers of the specimen is a necessary step towards 3D volume renderingof images. The Marching Cube Algorithm uses these layers of images tostack them up to form 3D images.

Marching cube algorithm is representative of 3D data field and usesvoxel unit to generate equal value surface technology. The usualalgorithm to process 3D positive data field, can be represented as:F _(i,j,k) =F(x _(i) , y _(i) , z _(k)) (i=1- - - , N _(x) ,J=1, - - - N_(y) ,k=1, - - - N=)

In the Marching Cube algorithm, voxel is a cubic body according to thelogic. It is formed by the four pixels from each of the two neighboringlayers which form a cube's eight corners. It is shown in FIG. 21.

This algorithm processes all the cubic voxel in the data field. First itcarries out classification of the corners of a cube. It separates outthe cube which intersects with the equal value surface. From theclassification of the top points, one establishes what that cubeoccupies in the index within the search and classification table. Asearch is carried out on the distribution model of the correspondingequal value surface within the classification table. One usesinterpolation to compute the intersection point of the equal valuesurface and the boundaries of the cube. From the corresponding positionsof each of the corners of the tube and the equal value surface, one usesthe intersection point of the equal value surface and the cubic bodyedge and follows a fixed rule to join up and form the equal valuesurface close to the triangular plate. One then uses interpolation tocalculate the value of the normal vector of the corners of thetriangular plate.

3. Drawing and Display

Open GL is a popular 3D imaging software interface suitable to use onthe PC platform to produce high quality 3D images. Open GL simplifiesthe drawing and display procedure. It transforms the views taken, themodel, the projection and visualization region and simplifies intospecial usage transformation coefficient, the elimination of hiddensurface problem is automatically completed by the interface Whenwithdrawing close up equal value surface triangular plates, itsimultaneously calculates the vector quantity of the corners of thetriangular plate. After introducing in illumination processing, then onecan obtain the real effect 3D images.

Ultrascan—1 software's visualization portion and display model possesesgood interaction properties. The parameters of viewing angles, sampleposition, light source position can be conveniently adjusted. Thisenables good quality 3D images.

4. The Results of the Application of Visualization Technology

Using the above visualization technology, one can display the 3D imagesof the structures of the samples. FIGS. 22, 23 and 24 show imagesproduced after inspection of a flipchip, ceramic base plate and ceramicstick.

A Φ 30×15 mm aluminum sample was also inspected. The result is shown inFIG. 25(a). The vertical section of the sample is quite thick. So,multi-reflections and useless fixed pulse influence will be present.During the data preprocessing, the. above factors have to be eliminatedand also one needs to compress the pulse width as a preprocessing step.Then one can obtain the equal value surface. The 3D image or the sampleis shown in FIG. 25(b).

Software is written with Visual C++. The operating system is Win98.

Software functions: main dialog, file, view and test menu. Alsoincluding position process, file saving, mechanical scan and datacollect, gain control signal process and image process, etc. The menustructure is shown in FIG. 26.

-   1. Under main menu: file, test, view and help submenu.-   2. Under file submenu: new, open, print, save and exit submenu.-   3. Under test submenu: system setting and start submenu. Testing    parameter are set and start scanning. Under side is A scan image and    upper left side is C scan image, upper right side is B scan image.-   4. When a file is opened under the file menu. There are file, test,    edit, view, image, window, help and display submenu. Under the edit    submenu, file can be copied. Under display submenu, there are    functions such as: grayscale, inverse grayscale, pseudo color, zoom    in, zoom out and note. Under image submenu, there are functions such    as: eliminate fixed target, reduce pulsewidth, and extract isolines,    3D image.

Advantages of this embodiment of the present invention include:

-   1. Increased data acquisition rate of one A/D card to such a level    that it can acquire in real time the waveform of an un-modulated    reflected wave. The function done by hardware is completed by    software and more functions are added. C scanning SAM with single    function is changed to multipurpose SAM with A, B, C image scan-and    three-dimensional image functions. It is also phase SAM with    software to test the phase.-   2. This equipment can do analysis with multi-parameters to complete    the characteristic and model identification. SAM is made as a king    of quantitative testing and analysing equipment.-   3. With advanced enhanced functions, hardware such as is    oscilloscope, modulator and video frequency amplifier are omitted.    The hardware structure is simplified a lot, reduced volume, weight    and low cost and price are benefits for extending this technologies    and equipment.-   4. This equipment with more software is more flexible and    extendable; make single functional fixed equipment developed    according to the requirement of customer.

1. An acoustic lens for an acoustic microscope, including an inner metal portion, an outer glass portion and an outer metal housing, the outer glass portion having at least one opening therein, which exposes the inner metal portion, the outer metal housing serving as an outer protective housing to the outer glass portion, the glass and metal of the inner portion and outer protective housing being fused.
 2. An acoustic lens according to claim 1, wherein the inner metal portion has a concave region at a position corresponding generally to the opening in the outer glass portion.
 3. An acoustic lens according to claim 2, wherein a piezoelectric layer is formed over said concave region.
 4. An acoustic lens according to claim 3, wherein a metallic film is formed over said piezoelectric layer, so as to form an electrode.
 5. An acoustic lens according to claim 4, wherein the metallic film comprises gold.
 6. An acoustic lens according to claim 5, wherein a protective layer is formed over said metallic layer.
 7. An acoustic lens according to claim 6, wherein said protective layer is a high-hardness film such as silicon nitrate.
 8. An acoustic lens according to claim 3, wherein said piezoelectric layer comprises one of zinc oxide, lithium niobate and silica.
 9. An acoustic lens according to claim 8, wherein a metallic film is formed over said piezoelectric layer, so as to form an electrode.
 10. An acoustic lens according to claim 9, wherein a protective layer is formed over said metallic layer.
 11. An acoustic lens according to claim 10, wherein said protective layer is a high-hardness film such as silicon nitrate.
 12. An acoustic lens according to claim 9, wherein the metallic film comprises gold.
 13. An acoustic lens according to claim 12, wherein a protective layer is formed over said metallic layer.
 14. An acoustic lens according to claim 13, wherein said protective layer is a high-hardness film such as silicon nitrate.
 15. An acoustic lens according to claim 2, wherein said piezoelectric layer comprises one of zinc oxide, lithium niobate and silica.
 16. An acoustic lens according to claim 15, wherein a metallic film is formed over said piezoelectric layer, so as to form an electrode.
 17. An acoustic lens according to claim 16, wherein a protective layer is formed over said metallic layer.
 18. An acoustic lens according to claim 17, wherein said protective layer is a high-hardness film such as silicon nitrate.
 19. An acoustic lens according to claim 18, wherein a protective housing is provided for said outer glass portion.
 20. An acoustic lens according to claim 19, wherein the glass and metal of the outer and inner portions are fused.
 21. An acoustic lens according to claim 18, wherein the glass and metal of the outer and inner portions are fused.
 22. An acoustic lens according to claim 1, wherein a protective housing is provided for said outer glass portion. 